Process for manufacturing fluoroolefins

ABSTRACT

A process for manufacturing fluoroolefins comprising perfluorinating a starting material comprising at least one carbon-bonded hydrogen by electrochemical fluorination, dissociating this separated effluent in a pyrolysis, quenching and separating the effluent to yield tetrafluoroethylene and/or hexafluoropropylene.

FIELD OF INVENTION

[0001] This invention relates to a process for manufacturingfluoroolefins. More particularly, the present invention relates to usingelectrochemical fluorination and a pyrolysis to manufacturetetrafluoroethylene and/or hexafluoropropylene.

BACKGROUND OF INVENTION

[0002] Tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) arewidely used as monomers in the manufacture of plastic and elastomericfluoropolymers. See, for example, J. Scheirs in Modern Fluoropolymers,Wiley, 1996. The worldwide consumption of TFE exceeds 10⁵ tons/year. HFPis used as a comonomer to manufacture thermoplastic and elastomericfluoropolymers and as starting material for making hexafluoropropeneoxide (HFPO). The worldwide consumption is estimated to be 30,000tons/year.

[0003] There are several known methods for manufacturing TFE and HFP.The most common method and almost exclusively used at industrial scale,involves pyrolyzing CHClF₂ (R-22). See for example, U.S. Pat. No.2,551,573. The high temperature (600° C. to 1000° C.) pyrolysis ofCHClF₂ yields TFE and HFP in high yields. But there are environmentalconcerns with R-22. This process produces equimolar amounts of aqueoushydrochloric acid and considerable amounts of partially fluorinated andchlorinated compounds, which are difficult to separate from TFE toobtain polymerization grade TFE (U.S. Pat. No. 4,898,645). For theaqueous hydrochloric acid, industrial applications are general soughtthat can use the aqueous hydrochloric acid. The fluorinated and otherside products have to be incinerated through thermal oxidizers, which isanother costly process and produces high amounts of CO₂.

[0004] U.S. Pat. No. 5,611,896 describes a process where elementalfluorine is reacted with carbon to produce CF₄, which is converted toTFE in a plasma torch in the presence of carbon. Unreacted CF₄ is fedback to the plasma. Thus, this technology is advantageously“closed-loop” which means emissions to the environment are minimal. Butthis process is hardly economically viable due to the use of costlyelemental fluorine and the high-energy consumption involved.

[0005] U.S. Pat. Nos. 5,633,414 and 5,684,218 describe a plasma process,where metal fluorides, particularly CaF₂ as a cost efficient fluorinesource, are reacted with carbon in a plasma. Thus, the costs forelemental fluorine are avoided. This technology still requireshigh-energy consumption.

[0006] A further method described in the art involves reacting TFEand/or HFP with ethylene and then fluorinating the cyclobutanes byelectrochemical fluorination (ECF). This perfluoro-cyclobutane productis then pyrolyzed using conventional pyrolizing techniques as describedfor example in EP 455,399 including the references cited therein and WO00/75092. Any by-products formed in the ECF process are separated offand are not further used in accordance with the teaching of WO 00/75092.Accordingly, substantial waste material is produced with this process,which causes an environmental burden and makes the process economicallyless attractive. Additionally, the process requires the use of TFE asone of the starting compounds, which creates an additional economicaldisadvantage as part of the TFE produced is needed to produce furtherTFE.

[0007] U.S. Pat. No. 3,081,245 discloses a process for preparing TFEthat comprises feeding a saturated perfluorocarbon to a continuouselectric arc, passing the emerging gaseous product through a carbon bedat a temperature of 2700° C. to 2000° C. and quenching the resultinggaseous product mixture to less than 500° C. in less than one second.

[0008] EP 371,747 discloses a process for making TFE by heating in thepresence of a gas selected from Ar, HF, CO, CF₄, and CO₂ at atemperature of at least 2000° K. a C₂ to C₁₀ compound containingfluorine and hydrogen in which the F to H ratio is greater than or equalto 1 and the F to C ratio is greater than or equal to 1. Heating iscarried out with a Direct Current (DC) plasma or through radio frequencyenergy.

[0009] Another chlorine-free process for making TFE is disclosed in GB766 324 by pyrolyzing a fluorocarbon with at least 3 carbons permolecule. Pyrolyzing occurs at a temperature of at least 1500° C.preferably generated in an electric arc. The side products of thepyrolysis are fed back in the pyrolysis furnace after the separation ofTFE. The fluorocarbons to be pyrolyzed are obtained from exhaustivefluorination of petroleum fractions using elemental fluorine, whichrenders the process economically unattractive.

[0010] Still another chlorine-free method to make TFE is described in EP0 647 607. Finely divided fluoropolymers such as PTFE or perfluoro- orhighly fluorinated copolymers are pyrolyzed with superheated steam. Thesource of this feedstock is scrap material that cannot be used, ormaterials from worn out equipment. This process is an economicalmanagement of waste material. Another chlorine-free process to make TFEis described in WO 01/58840-A2. Solid particulate fluorocarbons,particularly PTFE and highly or perfluorinated polymers are subjected toDC plasma to yield TFE. Still another chlorine-free process to make TFEis disclosed in WO 01/58841-A1 where gaseous or liquid fluorocarbons arepyrolyzed via DC plasma. Another chlorine-free process to make TFE isdescribed in WO 01/58584-A2. Gaseous, liquid, and solidperfluorocarbons, particularly perfluoropolymers, are pyrolyzed viainductive heating. These processes cannot replace the standardtechnology via R22, because the technology does not produce newC—F-bonds and therefore cannot meet the demand for TFE.

[0011] Thus, the need exists for a process to manufacture TFE and/or HFPthat is efficient, environmentally friendly, and/or cost efficient.

SUMMARY OF THE INVENTION

[0012] We have found a process for manufacturing TFE that may have anefficient yield (overall yield preferably is higher than 90% based on ahydrocarbon feed) and may eliminate a hydrochloric acid waste stream.The process of the present invention may also produce HFP and can thusbe used to make both TFE and HFP if desired. The process generallyinvolves less separation efforts to purify TFE, can be designed in acost efficient manner, and can be designed as a so-called closed-loop inwhich no or very low amounts of waste material is created. Thisclosed-loop process is environmentally advantageous.

[0013] The present invention provides a process for manufacturingtetrafluoroethylene and/or hexafluoropropylene comprising the steps of:

[0014] (a) perfluorinating a starting material comprising a linear orbranched hydrocarbon compound and/or a partially fluorinated linear orbranched hydrocarbon compound by electrochemical fluorination (ECF) inan electrochemical cell (ECF cell) in a solution of anhydrous liquidhydrogen fluoride under temperature and pressure conditions sufficientto replace all hydrogens in at least part of the starting material withfluorine, to yield an ECF effluent;

[0015] (b) separating said ECF effluent to yield a perfluorinated feedmaterial;

[0016] (c) pyrolyzing said perfluorinated feed material to yield areaction mixture;

[0017] (d) quenching said reaction mixture to yield a product mixture;and

[0018] (e) recovering tetrafluoroethylene and/or hexafluoropropylenefrom said product mixture.

[0019] According to one embodiment, the pyrolysis of step (c) is carriedout in the presence of carbon. This allows for the conversion ofperfluorinated compounds that have a high F to C ratio such as CF₄ andC₂F₆. These compounds are typically present in an off-gas stream of theECF cell and can in accordance with an embodiment of the presentinvention be separated therefrom and thus pyrolyzed in the presence ofcarbon. The pyrolysis may proceed with a DC plasma or through inductiveheating and is preferably carried out in the presence of carbon. Forinductive heating, the pyrolysis may be carried out at a temperature ofat least 500° C., generally from 500° C. to 3000° C. (inclusive),typically between 700° C. and 3000° C. (inclusive), or between 900° C.and 1500° C. (inclusive).

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows schematically one embodiment of the inventive processas a closed-loop. A hydrocarbon feedstock is electrochemicallyfluorinated in an ECF cell (10). The lower boiling fluorocarbons areseparated (11) from the off-gas, mainly hydrogen, stream 10 a, andoptionally further separated into perfluorinated compounds to be fed inthe pyrolysis furnace (20), stream 11 a, and partially fluorinatedcompounds to be fed back to the ECF cell (10), stream 11 b. The higherboiling fluorinated chemical compounds are separated from the ECFeffluent or so-called brine of the ECF cell (12), stream 10 b. Thesefluorinated compounds are further separated in perfluorinated compoundsto be fed as the perfluorinated feed material in the pyrolysis furnace(20) and partially fluorinated compounds that are fed back to the ECFcell (10), stream 12 b.

[0021] The perfluorinated compounds of stream 11 a and 12 a arepyrolyzed at temperatures of 500° C. to 3000° C. in a pyrolysis chamber(20) and quenched. The quenched gases, stream 20 a, are subjected todistillation (30) yielding TFE and optionally HFP, and undesiredby-products, which are fed back to the pyrolysis furnace (20), stream 30b.

[0022]FIG. 2 shows another embodiment of the present invention. Thestream 30 b of FIG. 1 is now fed into a DC plasma furnace (40), stream30 c. The quenched pyrolyzed gases are fed back to the distillation(30), stream 40 a, to recover TFE and optionally HFP therefrom. In thisembodiment, all or part of the perfluorinated compounds from the off-gasstream 10 a of the ECF cell may be fed in the DC plasma furnace (40), ascarrier gases (stream 11 c).

[0023] These figures are not to scale and are intended to be merelyillustrative and nonlimiting.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0024] The present invention provides a process for manufacturingtetrafluoroethylene and/or hexafluoropropylene. The process involvesperfluorinating a starting material using electrochemical fluorinationand then feeding the perfluorinated material into a pyrolysis furnace toyield TFE and optionally HFP.

[0025] The process of the present invention is preferably designed as aclosed-loop process where all perfluorinated compounds can be convertedinto fluoroolefins and all undesired byproducts (e.g., C—Hcontaining/partially fluorinated materials) can be recycled untilcompletion. This reduces the process cost and fluorinated compounds inwaste streams. Thus, the process of the present invention isenvironmentally responsible. A variety of hydrocarbons (linear,branched, saturated, unsaturated) can be fed into the ECF cell. Theperfluorinated ECF effluent is fed into the pyrolysis and the partiallyfluorinated materials are fed back to the ECF cell. In the pyrolysis,desired TFE and/or HFP is produced. Any perfluorinated waste productscan be recycled and again be subjected to pyrolysis. The pyrolysis ispreferably carried out in the presence of carbon. Inductive heating isadvantageously used but also DC plasma can be used. When inductiveheating is used, the pyrolysis can proceed at a temperature of at least500° C. and not more than 3000° C. Any partially fluorinated compoundsin the waste streams can be re-fed into the ECF cell.

[0026] Advantageously, the present invention requires less separationefforts than processes known in the art especially during the separationand purification of TFE. This reduces the costs involved. The capitaland energy costs are less because simple distillation is involved (fewercolumns) versus R-22 pyrolysis. In addition, the process of the presentinvention may be economically feasible even at a smaller volume scale(e.g., 1000 tons/year) than the processes known in the art, and thus mayrequire less capital expenditure. TFE cannot be efficiently transportedbecause of its instability. Therefore, TFE is typically converted to apolymer or further processed prior to transport. Thus, it isadvantageous to be able to produce TFE at the site where the end polymeris produced. Because the process of the present invention iseconomically feasible at low volume production, the TFE can be readilyproduced at the site where the end polymer is made.

[0027] “Fluorinated” refers to chemical compounds having at least onecarbon-bonded hydrogen replaced by a fluorine, and specifically includesperfluorinated compounds and partially fluorinated compounds, i.e.,compounds that have C—F and C—H bonds in the molecule.

[0028] “Perfluorinated” compounds refers to chemical compounds whereessentially all carbon-bonded hydrogens have been replaced by fluorines,although typically some residual hydride will be present in aperfluorinated composition; e.g., preferably less than 2 weight %perfluorinated product.

[0029] One embodiment of the process of the present invention is setforth in FIG. 1. In FIG. 1, HF and starting material are fed into theECF cell 10. The ECF effluent 10 b is then fed to the separation process12 and the off-gas from the ECF cell 10 a is fed to a membrane process11. The membrane process separates the off-gas 10 a, partiallyfluorinated compounds 11 b, and perfluorinated compounds 11 a. Thepartially fluorinated compounds are returned to the ECF cell for furtherprocessing. The off-gas (H₂) may be vented or used for energyproduction. The ECF effluent 10 b is separated 12 and the desiredperfluorinated compound 12 a and the perfluorinated compounds 11 a arefed to the pyrolysis furnace 20. After pyrolysis, the product mixture 20a is then fed into the separation process 30, which typically is asimple distillation. The desired products TFE and/or HFP are separatedout (30 a). The undesirable fluorine-containing products 30 b arereturned to the pyrolysis furnace for further processing.

[0030] In FIG. 2, perfluorinated compounds 11 c from the off-gas 10 aand the perfluorinated compounds from the distillation 30 (stream 30 c)are subjected to a DC plasma (40). The reaction mixture of the DC plasmais quenched and then fed into the separation process 30 (stream 40 a).All of the off-gas perfluorinated compounds may be fed into the DCplasma (stream 11 c) in which case stream 11 a will not be present oronly part thereof may be fed into the DC plasma such that both streams11 a and 11 c co-exist. Likewise, only stream 40 a may be used oralternatively co-exist with stream 30 b.

[0031] Starting Materials

[0032] A variety of materials can be used as the feedstock or thestarting materials for ECF. The feedstock material can be a gas, aliquid, or a mixture thereof. The feedstock material generally compriseslinear or branched hydrocarbon compounds, partially fluorinated linearor branched hydrocarbon compounds or mixtures thereof. The linear orbranched hydrocarbon compound generally consists of carbon and hydrogenbut hydrocarbon compounds having one or more substituents such ashydroxy, amino groups, carboxy groups, sulphonic acid groups and amidegroups are within the scope of the term ‘hydrocarbon compound’ as usedin this invention. Preferably, however the feedstock will besubstantially free of chlorine, bromine, or iodine containing materialsas these create undesirable waste material. “Substantially free” meansthat the feedstock material is either free of or contains a material inamount of not more than 1 or 2% by weight relative to the total weightof feedstock. The feedstock may contain cyclic compounds, such as cyclichydrocarbons in admixture with the linear or branched (partiallyfluorinated) hydrocarbon compounds. The process provides for the use ofmixtures of compounds as the feedstock and these mixtures may be complexin that they contain a large variety of different compounds.

[0033] Preferably, the starting material comprises a straight orbranched alkane that is entirely hydrocarbon (e.g., a straight chainalkane, C_(n)H_(2n+2), wherein n is from about 3 to 25, preferably fromabout 4 to 8 or 10, and more preferably n is 4 to 6), or, a partiallyfluorinated analog thereof (e.g., C_(n)H_(x)X_(y), wherein X isfluorine, and wherein x is at least 1 and x+y=2n+2). The hydrocarboncompound may comprise saturated and unsaturated compounds includingolefins and aromatic compounds such as benzene, toluene, or xylene.Examples of especially preferred starting materials include butane,pentane, hexane, heptane, and octane. Examples of preferred readilyavailable feed materials include methane and hydrocarbons up to C₁₀ andmixtures thereof, and mixtures of hydrocarbons with olefins (e.g.,isobutylene, etc.). A particular hydrocarbon feedstock includes crudeoil and petroleum fractions, so-called distillation cuts originatingfrom refining of crude oil and from making olefins such as ethylene andpropylene. Preferably, the boiling point of these petroleum fractions isnot more than 200° C., and more preferably not more than 150° C. or 100°C.

[0034] To keep the overall ECF cell pressure low, preferably the gaseousstarting material has a boiling point of at least −50° C. and is easy toliquefy, e.g., propane (b.p. −42° C.), propene (b.p. −47° C.), butane(b.p. 0° C.), butene (b.p. −6° C.), isobutylene (b.p. −7° C.). To ensurea fast and complete fluorination, the liquid starting materials arepreferably compounds having 10 carbon atoms or less; otherwise thefluorination proceeds slowly and extensive branching and fragmentationcan occur, which makes the separation step more difficult. Mixtures ofhydrocarbons and their isomers and olefins may be added to the ECF cellas starting materials.

[0035] Electrochemical Fluorination

[0036] Generally any electrochemical fluorination process can be used toperfluorinate the starting material. For example, the Simonselectrochemical fluorination process, the interrupted current process(see WO Publication 98/50603), the bipolar flow cell (see U.S. Pat. No.5,322,597), the SOLUTIA EHD process, and the like, may be used.

[0037] The Simons electrochemical fluorination (Simons ECF) process wascommercialized initially in the 1950s by Minnesota Mining andManufacturing Company. This ECF process comprises passing a directelectric current through an electrolyte, (i.e., a mixture offluorinatable organic starting compound, liquid anhydrous hydrogenfluoride, and perhaps a conductivity additive), to produce the desiredfluorinated compound or fluorochemical. Simons ECF cells typicallyutilize a monopolar electrode assembly, i.e., electrodes connected inparallel through electrode posts to a source of direct current at a lowvoltage (e.g., four to eight volts). Simons ECF cells are generallyundivided, single-compartment cells, i.e., the cells typically do notcontain anode or cathode compartments separated by a membrane ordiaphragm. The Simons ECF process is disclosed in U.S. Pat. No.2,519,983 (Simons) and is also described in some detail by J. Burdon andJ. C. Tatlow in Advances in Fluorine Chemistry (M. Stacey, J. C. Tatlow,and A. G. Sharpe, editors) Volume 1, pages 129-37, ButtersworthsScientific Publications, London (1960); by W. V. Childs, L. Christensen,F. W. Klink, and C. F. Kolpin in Organic Electrochemistry (H. Lund andM. M. Baizer, editors), Third Edition, pages 1103-12, Marcel Dekker,Inc., New York (1991); by A. J. Rudge in Industrial ElectrochemicalProcesses (A. T. Kuhn, editor), pages 71-75, Marcel Dekker, Inc., NewYork (1967); and by F. G. Drakesmith, Topics Curr. Chem., 193, 197,(1997).

[0038] Simons ECF can be carried out essentially as follows. A startingmaterial and an optional conductivity additive are dispersed ordissolved in anhydrous hydrogen fluoride to form an electrolytic“reaction solution.” One or more anodes and one or more cathodes areplaced in the reaction solution and an electric potential (voltage) isestablished between the anode(s) and cathode(s), causing electriccurrent to flow between the cathode and anode, through the reactionsolution, and resulting in an oxidation reaction (primarilyfluorination, i.e., replacement of one or more carbon-bonded hydrogenswith carbon-bonded fluorines) at the anode, and a reduction reaction(primarily hydrogen evolution) at the cathode. As used herein, “electriccurrent” refers to electric current in the conventional meaning of thephrase, the flow of electrons, and also refers to the flow of positivelyor negatively charged chemical species (ions). The Simons ECF process iswell known, and the subject of numerous technical publications. An earlypatent describing the Simons ECF process is U.S. Pat. No. 2,519,983(Simons), which contains a drawing of a Simons cell and itsappurtenances. A description and photograph of laboratory and pilotplant-scale electrochemical fluorination cells suitable for practicingthe Simons ECF process appear at pages 416-418 of Vol. 1 of “FluorineChemistry,” edited by J. H. Simons, published in 1950 by Academic Press,Inc., New York. U.S. Pat. No. 5,322,597 (Childs et al.) and U.S. Pat.No. 5,387,323 (Minday et al.) each refer to the Simons ECF process andSimons ECF cell.

[0039] Generally the Simons ECF process is practiced with a constantcurrent passed through the electrolyte; i.e., a constant voltage andconstant current flow. See for example W. V. Childs, et al., AnodicFluorination in Organic Electrochemistry, H. Lund and M. Baizer eds.,Marcel Dekker Inc., New York, 1991. The current passing through theelectrolyte causes one or more of the hydrogens of the starting materialto be to replaced by fluorine.

[0040] Various modifications and/or improvements have been introduced tothe Simons ECF process since the 1950s including, but not limited to,those described in U.S. Pat. No. 3,753,976 (Voss et al.); U.S. Pat. No.3,957,596 (Seto); U.S. Pat. No. 4,203,821 (Cramer et al.); U.S. Pat. No.4,406,768 (King); Japanese Patent Application No. 2-30785 (Tokuyama SodaK K); SU 1,666,581 (Gribel et al.); U.S. Pat. No. 4,139,447 (Faron etal.); and U.S. Pat. No. 4,950,370 (Tarancon).

[0041] Another useful electrochemical fluorination cell includes thetype generally known in the electrochemical fluorination art as a flowcell. Flow cells comprise a set (one of each), stack, or series ofanodes and cathodes, where reaction solution is caused to flow over thesurfaces of the anodes and cathodes using forced circulation. Thesetypes of flow cells are generally referred to as monopolar flow cells(having a single anode and a single cathode, optionally in the form ofmore than a single plate, as with a conventional electrochemicalfluorination cell), and, bipolar flow cells (having a series of anodesand cathodes).

[0042] U.S. Pat. No. 5,322,597 (Childs et al.) incorporated by referenceherein more recently describes the practice in a bipolar flow cell of anelectrochemical fluorination process comprising passing by forcedconvection a liquid mixture comprising anhydrous hydrogen fluoride andfluorinatable organic compound at a temperature and a pressure where asubstantially continuous liquid phase is maintained between theelectrodes of a bipolar electrode stack. The bipolar electrode stackcomprises a plurality of substantially parallel, spaced-apart electrodesmade of an electrically conductive material, e.g., nickel, which isessentially inert to anhydrous hydrogen fluoride and when used as ananode, is active for electrochemical fluorination. The electrodes of thestack are arranged in either a series or a series-parallel electricalconfiguration. The bipolar electrode stack has an applied voltagedifference that produces a direct current that can cause the productionof fluorinated organic compound.

[0043] Another example of a bipolar flow cell is the Solutia EHD(electrohydrodimerization) cell. See J. Electrochem. Soc.: REVIEWS ANDNEWS, D. E. Danly, 131(10), 435C-42C (1984) and Emerging Opportunitiesfor Electroorganic Processes, D. E. Danly, pages 132-36, Marcel Dekker,Inc., New York (1984).

[0044] In the interrupted current electrochemical fluorination processgenerally a reaction solution is prepared that comprises hydrogenfluoride and a starting material. The hydrogen fluoride is preferablyanhydrous hydrogen fluoride, meaning that it contains at most only aminor amount of water, e.g., less than about 1 weight percent (wt %)water, preferably less than about 0.1 weight percent water. The reactionsolution within the ECF cell includes an electrolyte phase comprising HFand an amount of starting material dissolved therein. In general, thestarting material is preferably to some degree soluble or dispersible inliquid hydrogen fluoride. Gaseous starting materials can be bubbledthrough the hydrogen fluoride to prepare the reaction solution, orcharged to the cell under pressure. Solid or liquid starting materialscan be dissolved or dispersed in the hydrogen fluoride. Startingmaterials that are relatively less soluble in hydrogen fluoride can beintroduced to the cell as a solute dissolved in a fluorochemical fluid.

[0045] The reaction solution is exposed to reaction conditions (e.g.,temperature, pressure, electric voltage, electric current, and power)sufficient to cause fluorination of the starting material. Reactionconditions chosen for a particular fluorination process depend onfactors such as the size and construction of the ECF cell, thecomposition of the reaction solution, the presence or absence of aconductivity additive, flow rate, etc.

[0046] The reaction temperature can be any temperature that allows auseful degree of fluorination of the starting material. The temperaturemay depend on the factors discussed in the preceding paragraph, as wellas the solubility of the starting material and the physical state of thestarting material or the fluorinated product.

[0047] The electricity passed through the reaction solution can be anyamount that will result in fluorination of the starting material. Thecurrent is preferably insufficient to cause excessive fragmentation ofthe starting material or to cause the liberation of fluorine gas duringfluorination.

[0048] The ECF effluent can be separated using conventional techniquessuch as distillation. The desired perfluorinated compounds are then fedto the pyrolysis. The insufficiently fluorinated compounds are returnedto the ECF cell for perfluorination.

[0049] The amounts of partially fluorinated materials in the feed to thepyrolysis (i.e., that still contain a C—H bond) preferably is less than10 weight percent, more preferably less than 5 weight percent, and mostpreferably less than 2 weight percent.

[0050] Membrane Process/Separation

[0051] The ECF cell may have one or more membrane systems to capture theoff-gas. Typically the off-gas is hydrogen (H₂). Somefluorine-containing compounds (i.e., perfluorinated andnon-perfluorinated compounds) are typically carried over by the off-gas.A membrane process can be used to capture the partially fluorinated andperfluorinated compounds and then the partially fluorinated compoundscan be fed back into the ECF cell. By introducing membrane separation,only H₂ is released from the overall process, advantageously resultingin a closed-loop process. The hydrogen gas released may find further usein generating energy for the process or to provide energy elsewhere in amanufacturing plant.

[0052] Membranes separate gases by the principle of selective permeationacross the membrane wall. For polymeric membranes, the rate ofpermeation of each gas is determined by its solubility in the membranematerial and the rate of diffusion through the molecular free volume inthe membrane wall. Gases that exhibit high solubility in the membraneand gases that are small in molecular size, permeate faster than larger,less soluble gases.

[0053] The output from the ECF process includes a large volume ofhydrogen, perfluorinated product, and partially fluorinated materials.The membrane process separates the hydrogen from the fluorinated speciesby allowing the smaller, more soluble hydrogen to pass through themembrane while concentrating the fluorinated material (permeate). Thedesire is to recover greater than 99% of the fluorinated materials atgreater than 99.9% purity (<<1% H2).

[0054] Suitable membranes are commercially available. One commerciallyavailable membrane is the MEDAL™ Gas-separation membrane available fromAir Liquide, Houston, Tex. (See also U.S. Pat. Nos. 5,858,065;5,919,285; 5,814,127; and 5,759,237.)

[0055] Alternatively, a cryogenic distillation process may be used toseparate the off-gas (H₂). In addition, catalytic “cold combustion” ofH₂ by metals (e.g., platinum) in the presence of O₂ can be used.

[0056] Pyrolysis

[0057] Pyrolysis is defined as subjecting perfluorinated materials e.g.,in a pyrolysis furnace ((20), FIG. 1), obtained from the ECF, streams 11a and 12 a, to temperatures above 500° C. thereby heat cracking theperfluorinated materials. The perfluorinated compounds can be fed in thefurnace as gases mostly under sub-atmospheric pressure. Theperfluorinated compounds fragment under these conditions prevailinglyinto difluoro carbenes ICF2.

[0058] The so obtained hot “reaction mixture” is subsequently quenched,i.e., rapid cooling to below 400° C., generally below 300° C. andpreferably below 100° C. typically within less than a second, preferablyin less than 0.1 seconds. Cooling rates of 10⁴-10⁵ K/sec may be used.These high cooling rates can be achieved either by conducting the hotreaction mixture through a bundle of pipes which are externally cooledor by injecting a coolant in the reaction mixture. The latter technologyis also called wet quenching, the former dry quenching. Cold gases orliquids, like liquid perfluorinated carbons or water can be used ascoolant. The efficiency of the quench process generally controls theselectivity of TFE. The higher the cooling rate the higher theselectivity and the less coking. Coking is formation of carbon arisingvia disproportionation of ICF₂ into carbon and CF₄. Coking interfereswith the quench process.

[0059] Heating of the pyrolysis chamber can be achieved from externalsources, like electric power or superheated steam. Typically, when usinginductive heating the pyrolysis is carried out at a temperature of atleast 500° C. to not more than 3000° C. A modern technology is inductiveheating via microwaves. The needed powerful microwave generators arecommercially available. Frequencies are usually at about 50 to 3000 kHz.Temperature is typically in the range of 600 to 3000° C., for example700° C. to 2500° C. Inductive heating via microwaves is described in WO95/21126, U.S. Pat. No. 5,110,996, WO 00/75092-A1, and WO 01/58584-A2.

[0060] Preferably, the pyrolysis proceeds in the presence of carbon.When inductive heating is used, carbon may be provided as a heat packingmaterial. The use of carbon is particularly advantageous to convert CF₄,C₂F₆ and other perfluorinated compounds where the ratio fluorine atomsto carbon atoms is significantly greater than 2, e.g., greater than 2.5or 2.7, to TFE. These compounds cannot be readily converted to TFE inabsence of carbon because of stoichiometric constraints. The neededtemperature to convert these perfluorinated compounds into TFE in thepresence of carbon can be readily achieved with inductive heating.

[0061] Another method for pyrolysis is the Direct Current (DC) Plasmatechnology as described for example in U.S. Pat. No. 5,611,896,incorporated by reference herein. A carrier gas is needed to maintainthe flame between the electrodes. Flame temperature may exceed 10000° K.Preferably, the DC plasma pyrolysis is also conducted in the presence ofcarbon. When CF₄ is a carrier gas, the CF₄ is also converted to TFE inaddition to other perfluorinated compounds that have a lower fluorine tocarbon ratio. Carbon may be provided for in the DC plasma pyrolysisthrough injection of powdery carbon or by operating “self consuming”carbon electrodes. The hot reaction mixture resulting in the DC plasmacan be quenched as described above to obtain TFE and/or HFP at highselectivities. Plasma technology is, for example, covered in “FluorineReactions in Plasma” by Barry Bronfin, MIT PRESS, Mass. (1967). Afurther suitable DC plasma installation is described for example in WO01/00156.

[0062] In a particular embodiment, pyrolysis at a temperature of notmore than 3000° C., e.g., via inductive heating is used to pyrolyzeperfluorinated compounds originating from the ECF effluent (FIG. 2,stream 12 a) and a DC plasma is used to pyrolyze perfluorinatedcompounds originating from the ECF off-gas (stream 11 c) anddistillation (stream 30 c). Preferably, each of these pyrolysis isconducted in the presence of carbon.

[0063] The process or part of it can be either batch or continuous. TheECF cell can produce a perfluorinated effluent that is fed batchwise tothe pyrolysis, or the ECF cell can produce a perfluorinated effluentthat is continuously fed into the pyrolysis. With either method, theprocess can be designed as a closed-loop.

[0064] Distillation

[0065] TFE and HFP are isolated from the quenched mixture of gases,streams 20 a and 40 a, via distillation (30). The mixture typicallycontains, TFE, HFP, perfluoroisobutylene (PFIB), and saturatedperfluoroalkanes like CF₄, C₂F₆, or octafluorocyclobutane. In contrastto the commonly used “chlorine” process via R22, hydrogen and chlorinecontaining chemical compounds are virtually absent. This renders theseparation of TFE and HFP via distillation relatively simple incomparison to the R22 process even when TFE is to be used in asubsequent polymerization to produce PTFE.

[0066] For making polymerization grade TFE, in particular for makingPTFE, hydrogen and chlorine-containing monomers like vinylfluoride,vinylidene fluoride, trifluorochloroethylene and the like preferably areremoved below the 1 ppm level because of their interference to make PTFEof desired quality and properties. Therefore, the existing processesrequire many distillation columns operated in complicated modes, as forexample detailed in DE 37 29 106-A1. As a consequence, only units with acapacity of several 1000 tons TFE/year have been economicallycompetitive.

[0067] The process of the present invention can yield polymerizationgrade TFE with only few distillation columns; essentially only 2 columnsare needed to separate the “low boiling” components like CF₄, C₂F₆, cyc.C₄F₈ from the high boiling components like PFIB. The distillation cutsof these side products are fed back to pyrolysis e.g., stream 30 b, FIG.1 to a low temperature pyrolysis using inductive heating, or in anotherembodiment of the present invention to a DC plasma furnace (40), stream30 c, FIG. 2.

[0068] Saturated perfluorinated components do not interfere at thepolymerization and thus can be tolerated as contaminations even athigher concentrations. The same holds for saturated “perfluoro” chemicalcompounds containing “isolated” hydrogen atoms. Isolated hydrogen isunderstood as a single hydrogen flanked by C—F-bonds. These hydrogensvirtually do not trigger chain transfer reactions at the polymerization.Thus, installation cost for the distillation are low. Smaller units witha capacity of less than 1000 tons TFE/year can thus be operatedeconomically.

EXAMPLES

[0069] The following examples illustrate various specific features,advantages, and other details of the invention. The particular materialsand amounts recited in these examples, as well as other conditions anddetails, should not be construed in a manner that would unduly limit thescope of this invention. All parts, percentages, and ratios are byweight unless otherwise specified.

Example 1 Simons Electrochemical Fluorination of Octane

[0070] A 1-liter electrochemical fluorination cell of the type describedin U.S. Pat. No. 2,567,011, equipped with 2 overhead condensers having anickel anode with a surface area of 0.037 m² was charged with 1000 gramsC₆F₁₄, 40 grams of dimethyldisulfide, 40 grams of octane and 200 gramsof anhydrous HF. The cell was operated at 45° C. and 2 bar. Voltage wasbetween 5-6 Volts, current density about 1500 A/m². Voltage was reducedfor 4 seconds to less than 4 Volts causing the current to fall toessentially zero after each 80 seconds (“intermitted current”). Octanewas continuously fed to the cell to maintain its concentration in thecirculating fluorochemical phase at about 5 wt %. Circulation rate wasvaried from 0.3 to 1 cell volume/hour with an external pump without anobservably distinct effect on the fluorination rates.

[0071] The experiment was run for 500 hours. Intermittently, a portionof the fluorochemical phase was removed, the perfluoroalkanes wereseparated, partially stored and partially recycled back to the ECF cell.HF was replenished according to current consumption. The off-gascontaining 1.7 volume % of fluorocarbons were stored in a vessel at 8bar and subjected to the Membrane Process set forth in Example 2 below.

[0072] The perfluoroalkanes were analyzed via gas chromatography forperfluoro octane. The yield of which was 15 wt %. The other products arelower fluoroalkanes fragmented down up to CF₄. Current efficiency wasabout 95%.

Example 2 Membrane Process

[0073] The off-gas stream emanating from the ECF cell behind theoverhead condensers at the run of Example 1 contained about 1.7 volume %of perfluorinated alkanes. Typical composition is shown in Table 1.TABLE 1 Composition of off-gas; balance H₂ Components CF₄ C₂F₆ C₃F₈C₄F₁₀ C₃F₁₂ CHF₃ Total Volume % 0.25 0.42 0.2 0.44 0.2 0.1 1.61

[0074] The off-gas stream was washed with aqueous NaOH solution,filtered to remove any liquid and solid particles compressed to 8 barand fed to a 2-stage membrane system consisting of a polyimide,asymmetric composite hollow fiber membrane. A MEDAL™ Gas separationprocess from Air Liquide, Houston, Tex., according to Example 4 of U.S.Pat. No. 5,814.127 was used. The output of the 2^(nd) membrane moduleyielded 99.9% fluorocarbon with less than 0.1% H₂, the composition ofwhich is given in Table 2. The “waste-stream” contained 99.7% hydrogen.The recovered fluorocarbons can be directly used as carrier gas at theDC plasma pyrolysis and also as feedstock for the inductive heatingpyrolysis. TABLE 2 Composition of the fluorocarbons separated from theECF-off-gas Components CF₄ C₂F₆ C₃F₈ C₄F₁₀ C₅F₁₂ CHF₃ H₂O HF Volume % 1626 12 27 12 6 ˜0.1 <1 ppm

Example 3 Preparation of TFE Via DC-Plasma Pyrolysis

[0075] A 30 kW DC plasma torch was used as described in WO 01/58841.Presence of carbon was given by “self-consuming” carbon electrodes. Dryquenching was used. The efficiency of this method is illustrated withpure fluorocarbons (Table 3) and the fluorocarbon mixture from theoff-gas of Example 2 (Table 4). The carrier gas used was CF₄. The CF₄stream was fed through a vaporizer containing the investigatedfluorocarbons. Flow rate was varied from about 3.5 to 7.5 kg/hr. Resultsare shown in Table 3. TABLE 3 DC plasma pyrolysis of selectedfluorocarbons Perfluorinated Feed Rate Flow Rate kg/hr Feed kg/hr ofquenched reaction mixture other Stock* Feed CF₄ CF₄ C₂F₄ C₃F₆ FC's C₈F₁₈(FC 3225) 0.45 3.0 1.7 1.2 0.15 0.4 C₇F₁₆ (PF 5070) 1.70 4.9 4.2 1.7 0.10.6 C₇F₁₆ (PF 5070) 1.53 3.0 2.4 1.3 0.0 0.8 C₆F₁₄ (PF 5060) 2.86 4.03.5 2.4 0.4 0.6 C₅F₁₂ (PF 5050) 4.40 3.0 3.7 3.7 0.0 <0.1

[0076] Table 4 gives the results of the fluorocarbon mixture asrecovered from the off-gas (Example 2). No additional CF₄ was used ascarrier gas. TABLE 4 DC plasma pyrolysis of fluorocarbon mixture fromoff-gas Flow Rate Flow Rate kg/hr of quenched reaction mixture ofoff-gas kg/hr fluorocarbons CF₄ C₂F₄ C₃F₆ Other FC's 4.5 2.5 1.8 0.0 0.27.5 3.1 4.3 0.0 0.1

Example 4 Preparation of TFE and HFP Via Inductive Heating Pyrolysis

[0077] A 10 kW installation as described in WO 01/58584-A2 was used.Frequency of microwaves was 800 kHz. Heat packing material used wasgraphite. Dry quenching was used. Perfluorooctane as model substance wasinvestigated. It was fed in the installation in gaseous state via avaporizer. Pressure was about 0.4 bar. Feed rate and average temperaturewas varied. Results are shown in Table 5. TABLE 5 Pyrolysis of C₈F₁₈ viainductive heating Temperature 700° C. 1000° C. Flow Rate Flow Rate ofkg/hr quenched reaction mixture of off-gas kg/hr fluorocarbons C₂F₄ C₃F₆C₂F₄ C₃F₆ 2 1.3 0.5 1.0 0.3 5 2.5 0.8 2.1 0.6 10 3.0 0.8 1.9 0.4

Example 5

[0078] The same installation as in Example 4 was used to investigate theconverting of CF₄ to TFE and HFP. Results are given in Table 6. CF₄ wasfed into the installation at a flow-rate of 2 kg/hr at varyingtemperatures of the heating packing material (graphite). The samequenching as in Example 4 was practiced. Results are given in Table 6.TABLE 6 Pyrolysis of CF₄; flow rate 2 kg/hr Temperature ° C. 900 20002500 Flow Rate kg/hr C₂F₄ C₃F₆ C₂F₄ C₃F₆ C₂F₄ C₃F₆ <0.1 <0.1 0.5 0.2 0.70.2

[0079] Table 6 illustrates that CF₄ can be converted to TFE and HFP atreasonable rates. The yield for these monomers suffers from decreasingquenching rates at higher pyrolysis temperatures.

What is claimed is:
 1. A process for the manufacture oftetrafluoroethylene and/or hexafluoropropylene comprising the steps of:(a) perfluorinating a starting material comprising a linear or branchedhydrocarbon compound and/or a partially fluorinated linear or branchedhydrocarbon compound by electrochemical fluorination (ECF) in anelectrochemical cell (ECF cell) in a solution of anhydrous liquidhydrogen fluoride under temperature and pressure conditions sufficientto replace all hydrogens in at least part of the starting material withfluorine, to yield an ECF effluent; (b) separating said ECF effluent toyield a perfluorinated feed material; (c) pyrolyzing said perfluorinatedfeed material to yield a reaction mixture; (d) quenching said reactionmixture to yield a product mixture; and (e) recoveringtetrafluoroethylene and/or hexafluoropropylene from said productmixture.
 2. The process according to claim 1, wherein said startingmaterial is a gas, a liquid, or a mixture thereof.
 3. The processaccording to claim 1, wherein the starting material comprises a straightor branched alkane represented by the formula: C_(n)H_(2n+2), wherein nis from about 3 to about 25 or an olefin.
 4. The process according toclaim 3, wherein n is from about 4 to about
 10. 5. The process accordingto claim 1, wherein the starting material is represented by the formula:C_(n)H_(x)X_(y) wherein X is fluorine, and wherein x is at least 1 andx+y=2n+2.
 6. The process according to claim 1, wherein the startingmaterial comprises butane, pentane, hexane, octane, or a mixture thereof7. The process according to claim 1, wherein the starting materialcomprises a petroleum fraction having a boiling point of not more than200° C.
 8. The process according to claim 1 wherein said startingmaterial is substantially free of chlorine, bromine, and/or iodinecontaining materials.
 9. The process according to claim 1, wherein saidECF process is Simons ECF, interrupted current ECF, or bipolar flow cellECF.
 10. The process according to claim 1, wherein said ECF effluent isseparated using simple distillation.
 11. The process according to claim1, wherein said perfluorinated feed material is pyrolyzed in thepresence of carbon.
 12. The process according to claim 1, wherein saidpyrolysis is carried out using inductive heating at a temperature of notmore than 3000° C. or using a DC plasma.
 13. The process according toclaim 1, wherein said ECF cell generates an off-gas and said processfurther comprises the step of separating said off-gas.
 14. The processaccording to claim 13, wherein partially fluorinated material isseparated from said off-gas and re-introduced in said ECF cell.
 15. Theprocess according to claim 13, wherein perfluorinated material isseparated from said off-gas and pyrolyzed in the presence of carbon at atemperature of not more than 3000° C.
 16. The process according to claim13, wherein perfluorinated material is separated from said off-gas andintroduced into a DC plasma as carrier gas together with waste productobtained in a distillation of said product mixture to recovertetrafluoroethylene and/or hexafluoropropylene therefrom.